Abstract:

Carbonatites and their surface weathering products constitute the main source of strategic metals, such as REE and Nb. Thus it is of great scientific significance to understand the extraordinary enrichment mechanism of strategic elements in carbonatites. Previous studies show that the mineralized carbonatites are closely associated with alkaline complex, indicating the parent magma is carbonated silicate magma which is dominated by carbonated nephelinite. Some experiments showed that both immiscibility and fractional crystallization could lead to carbonatite formation during the crustal evolution of parent magma. However, multicomponent experiments have clearly shown that liquid immiscibility always occurs prior to fractional crystallization of carbonate minerals, and thus immiscible separation of carbonatite from silicate melts plays an important role in the mineralization of strategic metals in carbonatites. However, in contrast to the fact that REE and Nb are naturally hosted in carbonatite, the immiscibility experiments showed that the strategic metals (REE and Nb) are preferentially partitioned into the silicate melt (alkaline silicate rocks). Although the partition coefficients of REE and Nb in carbonate-silicate melt is dependent on the water content, i.e., the degree of polymerization of the melt, the mechanism by which strategic metal elements (REE and Nb) are extremely enriched in carbonatites is enigmatic as the formation of mineralized carbonatites is mostly under anhydrous condition. Therefore, in order to elucidate the key controlling factors for the mineralization of critical metals in carbonatite, further studies should focus on whether other ligands, in addition to water, may influence the partition coefficient of critical metals between the immiscible carbonatitic and nephelinitic melts.

Key words: carbonatite, critical metals, liquid immiscibility, experimental petrology, partitioning coefficient